In the prior art, X-ray systems are mostly used for supporting interventional or diagnostic procedures on the heart or in the region of the heart, in particular for visualizing the instruments used. In this context the term “fluoroscopy” (radioscopy) refers to the recording of series of X-ray images which are displayed directly during the recording in real time, on a computer screen for example, and are intended to assist the operator or examiner in ensuring the correct positioning and movement of the instruments.
A problem with prior art fluoroscopy is that patient and operating staff are exposed to radiation. The standard image rate of 15 images per second obviously means that 15 X-ray irradiations are also performed per second, which, in the case of relatively long interventions in particular, constitutes a substantial radiation load for the patient and also, due to the necessary movements and inadequate X-ray protection associated therewith, for the examiner and the support staff, even when modern, sensitive X-ray devices are used. For this reason methods of minimizing the exposure to radiation have been sought since the introduction of fluoroscopy. A simple approach to reducing the exposure to radiation is to reduce the image rate. Thus, for example, compared with a standard image rate of 15 images per second, reducing the image rate to 3 images per second can reduce the exposure to radiation by a factor of five. However, an approach of this kind revealed itself as problematic, since the heart moves a great deal during the relatively long interval between the images and in addition the images occur in random cardiac phases, with the result that the series of images recorded sequentially in time becomes unstable and the image jerks more or less arbitrarily.
To overcome this problem it has been proposed to perform an ECG-triggered fluoroscopy in which the current cardiac phase is derived from the patient's ECG signal and only one fluoroscopy image per heartbeat is recorded. Typically, the R wave of the ECG signal is detected for this purpose and an image is recorded immediately after the R wave or after a selectable delay relative to the current R-R interval (for example at 30%). At a typical heart rate of between 60 and 80 beats per minute, this therefore results in approx. 1 to 1.3 images per second. The proposed method at least has the advantage that the images always originate from the same cardiac phase and therefore the image sequence is rendered considerably more stable. A significant problem with ECG-triggered fluoroscopy; however, is that the image rate, at approx. 1 image per second, is very low. A rapid movement of instruments, such as for example when advancing a guide wire, is practically impossible to control and monitor using this technology.
For this reason, finally, it has likewise been proposed in the prior art to record a plurality of images per heart cycle during ECG-triggered fluoroscopy. Thus, it is conceivable for example to record an image at the start and an image in the middle of the R-R interval. In this way the image rate can be increased to approx. 2 images per second.
With this approach, however, there is once again the problem that the two images may not necessarily originate from optically similar cardiac phases.
In addition there is the problem in practice of finding fixed trigger times, since the progression of the heart movement is influenced by many parameters, including, inter alia, the local position of the instrument in the heart, the angulation, the current heart rate, the individual pump function of the heart, and the individual anatomy.